Second-order nonlinear optical phenomena include many important processes that are widely used today. For example, optical second-harmonic generation is used to convert a laser light of a certain frequency to a light of twice the original frequency (i.e., one half the original wavelength). The linear electro-optical effect, another important example, is commonly used to modulate the phase of a light wave with an electric field.
Second-order nonlinear optical processes can only occur within media that do not possess inversion symmetry, i.e., where at least one polar axis whose positive and negative directions are different can be identified. In a molecular medium, the magnitude of a certain second-order optical nonlinearity is determined by both the corresponding molecular nonlinear optical polarizability (also called "hyperpolarizability") and the degree of the polar molecular alignment. Whether a molecular crystal possesses inversion symmetry or not depends critically on details of the molecular structure. A minor modification of the molecular structure can significantly alter the crystal's symmetry, as well as the molecular alignment. It is therefore difficult to simultaneously optimize both the hyperpolarizabilities and the polar alignment of molecules in a crystal. Furthermore, molecular crystals of high optical quality are often difficult to grow. Therefore, alternative routes for preparing molecular media that exhibit useful second-order nonlinear optical properties are highly desirable.
Since the mid-1980's, The Langmuir-Blodgett method has been recognized as a potential route for fabricating organic thin films for second-order nonlinear optics. The Langmuir-Blodgett method, invented during the 1930's, can be used to deposit thin films of certain organic compounds onto various substrates with the film thickness controlled to within a monomolecular layer. Normally, the compounds must be insoluble in water, as well as amphiphilic--containing both a hydrophilic (water-loving) terminal group and a hydrophobic (water-loathing) group at the opposite end. Such a compound can form a monomolecular layer on the water surface with the hydrophilic group anchored to water and the hydrophobic group pointing away from it, thereby leading to a polar alignment of the molecules. Typical examples of such monolayer-forming compounds are the fatty acid series, CH.sub.3 (CH.sub.2).sub.n COOH (n.gtoreq.13), where the carboxylic acid group is hydrophilic and the alkyl chain is hydrophobic. For nonlinear optical applications, an amphiphilic compound must also exhibit a large hyperpolarizability--achievable by the inclusion of a nonlinear optically active group in the molecule. Updated examples of nonlinear optical amphiphiles are given in the review article by B. Tieke, Adv. Mater. 2 222-231, (1990).
The majority of the reported Langmuir-Blodgett materials for second-order nonlinear optics are monomeric compounds. Recently use of multiple monolayers of polymeric and monomeric compounds have been reported. A second harmonic generation active Langmuir-Blodgett film consisting of alternate monolayers of a vinyl-maleic anhydride copolymer and a monomeric merocyanine dye has been reported by R. H. Tredgold et al., Electron. Lett. 24 308-309 (1988). They found a sub-quadratic dependence of the second harmonic generation intensity on film thickness. B. L. Anderson et al., Synth. Met. 28 D683-688 (1989) reported a new Langmuir-Blodgett film structure with each repeating unit consisting of two monolayers of hemicyanine-dye-grafted polyethers and two monolayers of a fatty acid--i.e., an ABCC-structure. The interlacing layers of the monomeric fatty acid were needed to obtain the desirable quadratic dependence of the second harmonic generation signal on the number of the polymeric nonlinear optical monolayers--for up to ten ABCC units. In virtually all known examples of nonlinear optical Langmuir-Blodgett materials, the hydrophobic groups contain straight long chains of hydrocarbons.
For most compounds synthesized to date for fabricating nonlinear optical Langmuir-Blodgett films, there exist several problems that can severely limit their practical uses: (1) the polar molecular alignment tends to degrade with increasing number of monolayers in the Langmuir Blodgett film; (2) the polar alignment induced during the film formation is not stable; (3) the optical quality of the Langmuir-Blodgett film is poor. Optical second harmonic generation is commonly used to characterize second-order nonlinear optical Langmuir-Blodgett films. Ideally, if the degree of the polar molecular alignment is the same within each monolayer, and if the Langmuir-Blodgett thickness is much less than an optical wavelength, the net second harmonic generation intensity from the entire Langmuir-Blodgett film should increase quadratically with the total number of nonlinear optically active layers. [See, for example, O. A. Aktsipetrov et al., Sov. Phys. JETP 61 524-530 (1985)]. However, in most known cases the increase in the second harmonic generation signal with film thickness is less than quadratic, presumably due to deteriorations of the polar alignment both with increasing film thickness and with time.
It is therefore an object of the present invention to provide a nonlinear optically active film in which the nonlinearity increases with the film thickness.
It is a further object of the present invention to provide a nonlinear optically active film comprising monolayers made of entirely polymeric compounds.
It is a further object of the present invention to provide novel polymers useful in fabricating nonlinear optically active films of improved optical quality and stability.